Multiple stressor effects of hypoxia and metals on ventilation, activity, bioaccumulation, and loss of equilibrium in fathead minnows.

Hypoxia is an increasing concern in freshwater ecosystems, driven by warming, stratification, and nutrient enrichment. Fish cope with hypoxia
through behavioural and ventilatory adjustments, yet growing evidence suggests that certain metals can impair these critical compensatory mechanisms. It is currently unknown whether lead (Pb) and nickel (Ni), released in part through expanding mining operations, may alter these critical survival responses. We tested whether 48 hr Pb and Ni exposure impaired fathead minnows’ (Pimephales promelas) responses to hypoxia
by measuring their gill metal accumulation, ventilation rates, behaviour (movement), and hypoxia tolerance (as measured by % DO at loss-of equilibrium (LOE)). Notably, Pb (43 μg/L) exposure during hypoxia resulted in 3.5 x greater gill bioaccumulation, indicating changes in gill uptake dynamics. Ni (318 μg/L) increased fish activity, as measured bytotal distance moved, in both normoxia and hypoxia. Opercularmovements were used as a proxy for the hypoxic ventilatory response (HVR) and the increase in ventilation in Ni exposed fish was 48% less than the increase in unexposed fish, indicating a blunted HVR. In the short term, both metals enhanced LOE tolerance. Therefore, Ni caused an increase in activity under hypoxia without the necessary ventilatory adjustments while maintaining hypoxia tolerance. Co-exposure to Pb and hypoxia may increase gill Pb accumulation due to a maintained HVR. Longer-term exposures across a broader range of concentrations would be needed to determine whether these responses persist or result in physiological trade-offs. These results underscore the importance of multiple-stressor research to better understand the interplay of hypoxia and metals to inform water quality guidelines.